Envelope detecting circuit

ABSTRACT

An envelope detecting circuit is for generating an envelope signal of an input RF signal described. The envelope detecting circuit includes an input terminal, an output terminal, a transistor, and an integrating circuit. The transistor, which is operated in the class B or the class C mode, receives the input RF signal from the input terminal, amplifies the input RF signal, and outputs an amplified signal. The integrating circuit, which is provided between the transistor and the output terminal, provides a series circuit of a resistor and a capacitor between the bias supply and the ground. The transistor receives the bias through the resistor. The capacitor holds bottom levels of the amplified signal.

BACKGROUND OF THE INVENTION 1. Field of the Invention

The present invention relates to an envelope detecting circuit that maygenerate an envelope signal of an input radio-frequency (RF) signal.

2. Related Background Art

It has been known that an envelope detecting circuit implements anactive element such as a field effect transistor (FET) where an FET isoperable as a diode. A Japanese Patent Application laid open No.JP-2000-068747A has disclosed such an envelope detecting circuit.However, when a transistor, which is a three-terminal device, is used asa diode, which is a two-terminal device, the transistor used thereinshows no function to amplify an input signal, which results in arestricted output power.

SUMMARY OF THE INVENTION

An aspect of the present invention relates to an envelope detectingcircuit that includes an input terminal, an output terminal, atransistor, and an integrating circuit. The input terminal receives aninput radio-frequency (RF) signal. The output terminal outputs anenvelope signal of the input RF signal. The transistor receives theinput RF signal in a control terminal thereof from the input terminal,amplifies thus received input RF signal, and outputs an amplified signalin one of current terminals. This one of the current terminals isconnected with the output terminal. The integrating circuit is providedbetween the transistor and the output terminal. The integrating circuitincludes a resistor and a capacitor connected in series between a biassupply and a ground. The transistor receives a bias in the one of thecurrent terminals from the bias supply through the resistor. A featureof the envelope detecting circuit of the preset invention is that thetransistor is operated in the class B mode or the class C mode.

Another envelope detecting circuit of the present invention provides aninput terminal, an output terminal, a balun, and two envelope detectingunits. The input terminal receives an input RF signal. The outputterminal outputs an envelope signal of the input RF signal. The balunreceives the input RF signal from the input terminal and generates twoinput signals complementary to each other. The two envelope detectingunits each receive the input signals from the balun and coupling theenvelope signals generated therein in the output terminal. The envelopegenerating units each provide a transistor and an integrating circuit.The transistor receives the input signal from the balun, amplifies theinput signal, and outputs thus amplified signal in one of the currentterminals thereof that is connected with the output terminal. Thetransistor is operated in the class B or the class C mode. Theintegrating circuit, which is provided between the transistor and theoutput terminal, includes a resistor and a capacitor connected in seriesbetween a bias supply and a ground. The transistor is biased in the oneof the current terminal thereof by the bias supply through the resistor.

BRIEF DESCRIPTION OF THE DRAWINGS

The foregoing and other purposes, aspects and advantages will be betterunderstood from the following detailed description of a preferredembodiment of the invention with reference to the drawings, in which:

FIG. 1 shows a circuit diagram of an envelope detecting circuitaccording to the first embodiment of the present invention;

FIG. 2 compares signal forms of the envelope detecting circuit havingthe integrating circuit with that without the integrating circuit;

FIG. 3A shows a signal form of an envelope detecting circuit when atransistor is operated in the class A mode, and FIG. 3B shows a signalform of an envelope detecting circuit when a transistor is operated inthe class AB mode;

FIG. 4 shows a circuit diagram of another envelope detecting circuitaccording to the second embodiment of the present invention;

FIG. 5 shows a circuit diagram of still another envelope detectingcircuit that is modified from the envelope detecting circuit shown inFIG. 4;

FIG. 6A compares signal forms of the envelope signals output from thecircuit shown in FIG. 4 with that output from the circuit shown in FIG.5, and FIG. 6B magnifies the envelope signals shown in FIG. 6A; and

FIG. 7 shows a circuit diagram of an envelope detecting circuitaccording to the third embodiment of the present invention.

DESCRIPTION OF EMBODIMENT

Next, embodiment according to the present invention will be described asreferring to accompanying drawings. In the description of the drawings,numerals or symbols same with or similar to each other will refer toelements same with or similar to each other without duplicatingexplanations.

First Embodiment

FIG. 1 shows a circuit diagram of an envelope detecting circuitaccording to the first embodiment of the present invention. The envelopedetecting circuit 100 provides a field effect transistor (FET) 10 whosegate is connected to an input terminal IN thorough an input matchingcircuit 12 that includes a capacitor C₂ and distributed transmissionlines, L₁ to L₃. Provided between the input terminal IN and the gate ofthe FET 10 is a series circuit of the capacitor C₂ and two distributedtransmission lines, L₁ and L₂. The distributed transmission line L₃ isconnected as an open stub between two distributed transmission lines, L₁and L₂, which is denoted as a node N₂. The capacitor C₂ may operate as acoupling capacitor to cut a DC component. The input matching circuit 12may match input impedance of the FET 10 with transmission impedance ofan external circuit that is not illustrated in figures. That is, thematching circuit 12 makes input impedance of the envelope detectingcircuit 100 viewed from the input terminal IN to be matched with thetransmission impedance of the external circuit.

A power supply 16 may provide gate bias of the FET 10 through a resistorR₂, the node N₂, and the distributed transmission line L₂. The resistorR₂ may prevent high frequency components from leaking to the powersupply 16.

The FET 10 in a source thereof is grounded, and a drain is connectedwith an output terminal OUT through a capacitor C₃ that is also operableas a coupling capacitor to cut a DC component. A voltage V_(d) issupplied to a node N₁ between the FET 10 and the output terminal OUTfrom a voltage source 14 through a resistor R₁. This node N₁ is alsogrounded through a capacitor C₁. The resistor R₁ and the capacitor C₁forms an integration circuit 50 with a time constant. The input terminalIN receives a radio-frequency (RF) signal 30 in a submillimeter bandand/or a millimeter band, while, the output terminal OUT outputs anenvelope signal 32 of this input RF signal 30.

The resistors, R₁ and R₂, have resistance of 400 Ω and 1 kΩ, thecapacitors, C₁ to C₃, have capacitance of 0.485 pF, 0.453 pF, and 2.2pF, respectively, and the distributed transmission lines, L₁ to L₃, haveelectrical lengths of 0.00595 λ, 0.0923 λ, and 0.0893 λ, where λ is awavelength of an input RF signal subject to the present envelopedetecting circuit 100. The FET 10 receives a drain bias V_(d) of 2 V anda gate bias V_(g) of 0 V.

Operation of Envelope Detecting Circuit

FIG. 2 schematically shows an output signal form of the FET 10 when theFET 10 is biased in the class B mode, that is, the FET 10 turns off forno input signal. Also, the FET 10 is further preferably biased in theclass C mode at which the FET 10 is further deeply biased compared withthe class B mode, that is, a period the FET 10 is turned off in theclass C mode becomes longer compared with a status when the FET 10 isbiased in the claims B mode. As shown in FIG. 2, when the input RFsignal 30 is 0 at t₁, the output of the FET 10, namely, a drain level ofthe FET 10 becomes equal to the drain bias V_(d) because the FET 10 isturned off and no drain current I₂ flows therein. Thereafter, when theinput RF signal 30 gradually increases from t₁ to t₂, the FET 10gradually turns on, the drain current I₂ thereof increases, and carriersstored in the capacitor C₁ discharges as the drain current I₂.

Thereafter, the input RF signal 30 gradually decreases from t₂ to t₃,the drain current I₂ decreases, during which the voltage source 14continuously supplies the current I₁ to the node N₁, which is going tokeep a voltage drop caused in the resistor R₁, that is, the node N₁ isgoing to lower the level thereof, however, a portion of the current I₁flows in the capacitor C₁ and charges thereof. Accordingly, the node N₁in the level thereof is substantially unchanged from the level at t₂.

From t₃ to t₄, the input RF signal 30 becomes less than 0 V, which turnsoff the FET 10 and cuts the drain current I₂, the current I₁ flows inthe resistor R₁ depending on a voltage difference between the level atthe node N₁ and the voltage V_(d) of the bias supply 14 and the timeconstant defined by a product of the resistance of the resistor R₁ andthe capacitance of the capacitor C₁. Setting the time constant longerthan a period of the input RF signal 30, the node N₁ in the voltagelevel thereof becomes gradually increases because the capacitor C₁ ischarged through the resistor R₁. Thus, the level at the node N₁substantially reflects the envelope of the signal amplified by the FET10.

According to the first embodiment, the FET 10, which operates in theclass B mode, receives the input RF signal 30 in the control terminalthereof. The FET 10 is ground in the source thereof and the drain isconnected with the output terminal OUT through the capacitor C₃.Provided between the FET 10 and the output terminal 10 is theintegrating circuit 50 that includes the resistor R₁ and the capacitorC₁ connected in series between the bias source 14 and the ground. TheFET 10 in the drain thereof is biased by the bias source 14 through theresistor R₁, while, it is grounded through the capacitor C₁.

According to the configuration above, the envelope detecting circuit 100may detect the envelope of the input RF signal 30 but amplified by theFET 10. Because the FET 10 may inversely amplify the input RF signal 30,the output of the envelope detecting circuit 100 may secure the outputpower therefrom. When the input RF signal 30 is in the submillimeterband, or the millimeter band, various reasons, such as impedancemismatching and/or losses in transmission lines, may increase signallosses, an envelope detecting circuit is strongly requested to suppressor compensate losses in the output thereof.

The envelope detecting circuit 100 of the embodiment preferably has agreater time constant to charge the capacitor C₁ by the current I₁ inorder to for in the envelope signal. The time constant for charging thecapacitor C₁ may be primarily determined by a product of the resistanceof the resistor R₁ with the capacitance of the capacitor C₁; and thetime constant in the present embodiment is preferably twice of a periodof the input RF signal 30, or further preferably five times greater thanthe period of the input RF signal 30.

FIGS. 3A and 3B schematically illustrates operations of the FET 10 inthe class A, FIG. 3A, and in the class AB, FIG. 3B. In the class A mode,the FET 10 never turns off, which means that the output of the FET 10without the integrating circuit 50 becomes a sine curve. Thus, the FET10 always flows the current I₂ in the class A mode, which sets theoutput signal in an average thereof to be half of the bias, namely,V_(d)/2. Also, the output signal becomes a sine wave but amplitudethereof is attenuated because of the existence of the capacitor C₁.Thus, the transistor operating in the class A mode may not detect theenvelope of the input RF signal 30.

When the FET 10 is operated in the class AB mode, the transistor mayturn off but a period thereof is shorter compared with the case of theclass B or the class C mode. The charge stored in the capacitor C₁ maydischarge through the FET 10 when the FET 10 turns on, which reduces thecurrent I₁ flowing in the resistor R₁ and raises the level at the nodeN₁. Thus, the class AB mode suppresses the amplitude of the outputenvelope signal.

Second Embodiment

FIG. 4 shows a circuit diagram of another envelope detecting circuitaccording to the second embodiment of the present invention. Theenvelope detecting circuit 102 includes a balun 20 and two envelopedetecting units, 22 and 24. The balun 20 generates two signalscomplementary to each other by receiving the input RF signal 30 form theinput terminal IN. The envelope detecting units, 22 and 24, haveconfigurations same with that shown in FIG. 1, where the integrationcircuit 52 a in the former envelope detecting unit 22 includes theseries circuit of the resistor R₁ and the capacitor C₁, while, that inthe latter envelope detecting unit 24 also includes the series circuitof the resistor R₁ and the capacitor C₁. That is the integrationcircuits, 52 a and 52 b, have the time constant same with each other.The outputs of the two envelope detecting units, 22 and 24, namely, thedrains of the respective FETs 10, are coupled with the node N₃ and thecoupling capacitor C₃ connected with the output terminal OUT is commonlyowned by the two envelope detecting units, 22 and 24. The envelopesignals generated in the respective detecting units, 22 and 24, aremerged at the node N₃ and output from the output terminal OUT as theenvelope signal 32. Details of the two envelope detecting units, 22 and24, namely, the operation of the two envelope detecting units, 22 and24, are same with that shown in FIG. 1. One of the outputs 30 a of thebalun 20 enters the gate of the FET 10 in the first envelope detectingunits 22, while, the other of the outputs 30 b enters the gate of theFET 10 in the other envelope detecting unit 24, each through the inputmatching unit 12.

FIG. 5 shows a circuit diagram of an envelope detecting circuit 102Amodified from that shown in FIG. 4. The circuit 102A of FIG. 5 has afeature distinguishable from that shown in FIG. 4 that the outputs ofthe respective envelope detecting units, 22 and 24, are independentlyprovided to the output terminals, OUT₁ and OUT₂. That is, the envelopedetecting units 22 may generate the first output in the terminal OUT₁through the coupling capacitor C₃₁ as the first envelope signal 38 a;while, the other envelope detecting units 24 may also generate thesecond output in the terminal OUT₂ through the coupling capacitor C₃₂ asthe second envelope signal 38 b.

Operation of Envelope Detecting Circuit in the Second Embodiment

FIG. 6A schematically illustrates the envelope signals, 38 a and 38 b,output from the respective detecting units, 22 and 24, shown in FIG. 5.Because the respective detecting units, 22 and 24, provides theintegration circuits, 52 a and 52 b, in the drains of the FET 10, thedetecting units, 22 and 24, may generate the envelope signals, 38 a and38 b, with the phase difference of π. The envelope detecting circuit 102of the second embodiment may combine these two envelope signals, 38 aand 38 b, at the node N₃, the output of the envelope detecting circuit102 becomes that 32 shown in FIG. 6B, that is, the output 32 of theenvelope detecting circuit 102 becomes an average of the two outputs, 38a and 38 b. The output 32 primarily contains a frequency component thatis twice of the frequency component of the input RF signal 30 and thefluctuation appearing in the output becomes half of those, 38 a and 38b, for a case that the envelope detecting function is carried out in thesingle phase by the circuit shown in FIG. 1. The fluctuation appearingin the output is sometimes called as a ripple.

According to the second embodiment, the balun 20 may generate twosignals, 30 a and 30 b, complementary to each other from the input RFsignal 30 each provided to the detecting units, 22 and 24. The envelopedetecting units, 22 and 24, have arrangements same with each other, thatis, the envelope detecting units, 22 and 24, include the FET 10 and theintegration circuit comprising the resistor R₁ and the capacitor C₁connected in series between the bias source 14 and the ground, andcoupled with the drain of the FET 10. Accordingly, the envelopedetecting circuit 102 may generate an envelope signal with a primaryfrequency component that is double of the primary frequency component ofthe input RF signal 30. The FET 10 in the respective envelope detectingunits, 22 and 24, may be preferably operated in the class B mode, orfurther preferably in the class C mode. The time constant of theintegrating circuit, 52 a and 52 b, is preferably greater than a periodof the input RF signal 30.

The description above concentrates in the arrangement where the envelopedetecting circuits, 100, 102, and 102A, provides a field effecttransistor (FET) as an amplifying element. However, the amplifyingelement is not restricted to an FET, a bipolar transistor and the likemay be implemented in the envelope detecting circuits, 100, 102, and102A.

Third Embodiment

FIG. 7 shows a circuit diagram of an envelope detecting circuitaccording to the third embodiment of the present invention. The envelopedetecting circuit 104 shown in FIG. 7 has a feature distinguishable fromthose of the aforementioned embodiment is that the circuit 104 providesa filter between the integrating circuit 10 and the output terminal OUT.The filter includes an inductor L₄ and a capacitor C₄, that is, thefilter operates as a low-pass filter, or a high-cut filter. Although theenvelope signal generated by the envelope detecting circuits, 100 to104, contains high frequency components including a primary frequencycomponent of the input RF signal 30 and a frequency component double ofthe primary frequency component in the envelope detecting circuit 102 ofthe second embodiment, the primary frequency component of the envelopesignal to be utilized is low frequency far lower than the primaryfrequency component of the input RF signal 30. The filter disposed inthe output of the FET 10 may effectively eliminate high frequencycomponents and make the envelope signal stable enough.

While, particular embodiment of the present invention has been describedherein for purposes of illustration, many modifications and changes willbecome apparent to those skilled in the art. Accordingly, the appendedclaims are intended to encompass all such modifications and changes asfall within the true spirit and scope of this invention.

The present application claims the benefit of priority of JapanesePatent Application No. 2016-205847, filed on Oct. 20, 2016, which isincorporated herein by reference.

I claim:
 1. An envelope detecting circuit, comprising: an input terminalthat receives an input radio-frequency (RF) signal; an output terminalthat outputs an envelope signal of the input RF signal; a transistorthat receives the input RF signal in a control terminal thereof from theinput terminal, amplifies the input RF signal, and outputs an amplifiedsignal in one of current terminals that is connected with the outputterminal; and an integrating circuit provided between the transistor andthe output terminal, the integrating circuit including a resistor and acapacitor connected in series between a bias supply and a ground, thetransistor receiving a bias in the one of the current terminals from thebias supply through the resistor, wherein the transistor is operated ina class B mode or a class C mode.
 2. The envelope detecting circuit ofclaim 1, wherein the integrating circuit has a time constant greaterthan a period of the input RF signal.
 3. The envelope detecting circuitof claim 1, wherein the transistor is operated in the class C mode. 4.The envelope detecting circuit of claim
 1. further including a low-passfilter provided between the transistor and the output terminal, thelow-pass filter eliminating a primary frequency component of the inputRF signal.
 5. The envelope detecting circuit of claim 4, wherein thelow-pass filter includes the capacitor.
 6. An envelope detecting circuitcomprising: an input terminal that receive an input radio-frequency (RF)signal; an output terminal that outputs an envelope signal of the inputRF signal; a balun that receives the input RF signal from the inputterminal and generates two input signals complementary to each other;and two envelope detecting units each receiving the input signals fromthe balun and generating envelope signal in the output terminal, whereinthe envelope detecting units each provide a transistor that receives theinput signal from the balun, amplifies the input signal, and outputs anamplified signal in one of current terminals that is connected with theoutput terminal, the transistor being operated in a class B or a class Cmode, an integrating circuit provided between the transistor and theoutput terminal, the integrating circuit including a resistor and acapacitor connected in series between a bias supply and a ground, thetransistor being biased in the one of the current terminals by the biassupply through the resistor.
 7. The envelope detecting circuit of claim6, wherein the integrating circuit has a time constant greater than aperiod of the input RF signal.
 8. The envelope detecting circuit ofclaim 6, wherein the transistor in the envelope detecting unit isoperated in the class C mode.
 9. The envelope detecting circuit of claim6. further including a low-pass filter provided between the transistorand the output terminal, the low-pass filter eliminating a primaryfrequency component of the input RF signal and a double of the primaryfrequency component.
 10. The envelope detecting circuit of claim 9,wherein the low-pass filter includes the capacitor.